Part I. Introduction: 1. Homosexuality in animals. Topics, hypotheses, and research trajectories Volker Sommer and Paul L. Vasey; Part II. Non-primates: 2. Making the best of a bad situation. Homosociality in male greylag geese Kurt Kotrschal, Josef Hemetsberger and Brigitte Weiss; 3. Pink flamingos. Atypical partnerships and sexual activity in colonially breeding birds Catherine E. King; 4. Establishing trust. Sociosexual behaviour and the development of male-male bonds among Indian Ocean bottlenose dolphins Janet Mann; 5. Going with the herd. Same-sex interaction and competition in American bison Hilde Vervaecke and Catherine Roden; 6. Exciting ungulates. Male-male mounting in fallow, white-tailed and red deer Ludek Bartos and Jana Holeckova; 7. Frustrated felines. Sexual mounting between males in feral cats Akihiro Yamane; Part III. Primates: 8. The pursuit of pleasure. An evolutionary history of homosexual behaviour in Japanese macaques Paul L. Vasey; 9. Getting to know you. Female-female consortships in free-ranging rhesus monkeys Ellen Kapsalis and Rodney L. Johnson; 10. A wild mixture of motivations. Same-sex mounting in Indian langur monkeys Volker Sommer, Peter Schauer and Diana Kyriazis; 11. Playful encounters. The development of homosexual behavior in male mountain gorillas Juichi Yamagiwa; 12. Social grease for females? Same-sex genital contacts in wild bonobos Barbara Fruth and Gottfried Hohmann; 13. The evolution of male homosexuality and its implications for human psychological and cultural variations Dennis Werner; Part IV. Outlooks: Science and Beyond: 14. Where do we go from here? Research on the evolution of homosexual behaviour in animals Paul L. Vasey; 15. Against nature?! An epilogue about animal sex and the moral dimension Volker Sommer; Acknowledgements.